1. Field of the Invention
The present invention relates to an optical waveguide element. In particular, the invention relates to a polarization conversion device that is a device for performing polarization conversion for a substrate-type optical waveguide element used in optical fiber communication.
Priority is claimed on Japanese Patent Application No. 2013-135491, filed on Jun. 27, 2013, the content of which is incorporated herein by reference.
2. Description of Related Art
In recent years, the amount of information used in optical communication has been increasing. In order to respond to such an increase in the amount of information, the measures have been taken for an increase in the signal speed, an increase in the number of channels by using wavelength division multiplexing, and the like. In particular, in digital coherent technology of the next generation 100 Gbps (gigabit per second) for high-speed information communication, in order to double the amount of information per unit time, a polarization multiplexing scheme for carrying information in each of two polarized waves having electric fields perpendicular to each other is used. However, modulation schemes for high-speed communication including the polarization multiplexing scheme require a complex optical modulator. For this reason, problems, such as a device size increase and a cost increase, occur. In order to solve such problems, an optical modulator having a substrate-type optical waveguide using silicon, which is advantageous in terms of easy processing, size reduction by integration, and cost reduction by mass production, has been studied.
However, the polarization multiplexing in the substrate-type optical waveguide has the following problems. In general, the substrate-type optical waveguide has a shape in which the width direction parallel to the substrate and the height direction perpendicular to the substrate are asymmetrical. For this reason, in two types of polarization modes of a mode in which an electric field component in the width direction is a main component (hereinafter, referred to as a TE mode) and a mode in which an electric field component in the height direction is a main component (hereinafter, referred to as a TM mode), the characteristics, such as an effective refractive index, are different. In many cases, a fundamental TE mode and a fundamental TM mode of these modes are used. Here, the fundamental TE mode refers to a mode having a largest effective refractive index of the TE modes, and the fundamental TM mode refers to a mode having a largest effective refractive index of the TM modes.
It is difficult to perform an optical modulation operation for these modes having different characteristics with a single substrate-type optical waveguide element. When using a substrate-type optical waveguide element optimized for each mode, in order to develop a substrate-type optical waveguide element for each mode, a lot of effort is required in terms of development.
As a method for solving this problem, a method can be mentioned in which a substrate-type optical waveguide element optimized for a desired fundamental TE mode is manufactured, the fundamental TE mode is used as the input, and the output is polarization-converted to the fundamental TM mode. The polarization conversion herein indicates a conversion from the fundamental TE mode to the fundamental TM mode or a conversion from the fundamental TM mode to the fundamental TE mode. In order to perform a polarization conversion operation, a substrate-type optical waveguide element for performing polarization conversion on the substrate is required.
As a technique for performing such polarization conversion on the substrate, there is a method of performing polarization conversion from the fundamental TE mode to the fundamental TM mode by coupling the fundamental TM mode input to one waveguide of a directional coupler to the fundamental TE mode of the other waveguide. Such a known technique is disclosed in Liu Liu, et al., “Silicon-on-insulator polarization splitting and rotating device for polarization diversity circuits,” Optics Express, Vol. 19, No. 13, pp. 12646-12651 (2011) (hereinafter, referred to as Non-patent Document 1). In Non-patent Document 1, a fundamental TM mode is input to one waveguide, and a fundamental TE mode is coupled to the other waveguide. Embodiments of the invention to be described later are not limited to the process of converting the fundamental TM mode to the fundamental TE mode but are focused on the reverse process, that is, on a structure of outputting the input fundamental TE mode as the fundamental TM mode. This is the same phenomena in passive waveguide elements.
The optical waveguide element shown in FIG. 1 of Non-patent Document 1 is a directional coupler and is formed by two parallel waveguides having the same height and different widths. In the directional coupler, a cross section (refractive index cross section) perpendicular to the longitudinal direction of each waveguide is formed by a lower clad, a rectangular core formed on the lower clad, and an upper clad that covers the lower clad and the core. The directional coupler of Non-patent Document 1 has a vertically asymmetrical structure in which the lower clad and the upper clad have different refractive indices. Each waveguide width is set such that the effective refractive index of the fundamental TE mode guided through the narrow waveguide and the effective refractive index of the fundamental TM mode guided through the wide waveguide become sufficiently close values.
When the refractive index cross section is vertically symmetrical, the amplitude direction of the main electric field of the fundamental TE mode is perpendicular to that of the fundamental TM mode. Therefore, in order to realize coupling using a directional coupler, it is required that the coupling length be sufficiently large and the effective refractive indices of the two modes be exactly the same. For this reason, in Non-patent Document 1, the upper and lower sides of the refractive index cross sections are made to be asymmetrical. In this case, a light confinement direction is tilted, and the number of components perpendicular to the main electric field component is increased for each mode. This enhances the coupling between the fundamental TE mode and the fundamental TM mode.
In order to make the upper and lower sides of the refractive index cross section asymmetrical, materials having different refractive indices are required for the upper clad and the lower clad. The need for new materials is disadvantageous in terms of effectiveness and cost because an extra process may occur or materials, which are not used in other optical waveguide portions originally, may be required.
In Non-patent Document 1, by using air for the upper clad and silica (SiO2) for the lower clad, “requiring no additional and nonstandard fabrication steps” is realized. In this case, since the optical waveguide is exposed during the manufacturing process, the characteristics are degraded due to adhesion of foreign matter. As a result, the yield is reduced.